There is a growing demand for high temperature and high performance polymers. It is particularly desirable to be able to control the molecular orientation of such polymers and to tailor the coefficient of thermal expansion (CTE) to optimize properties.
Polymers having improved properties have been obtained by the incorporation of reinforcing fibers, such as, glass, carbon and aramid, to form fiber reinforced polymers. However, disadvantages well known to those of ordinary skill in the art accompany the use of each of these reinforcing fibers.
Performance gains over fiber reinforced polymers have been achieved by blending thermoplastic flexible polymers with thermotropic rigid-rod polymers which are also called thermotropic liquid crystalline polymers (TLCPs). These blends are sometimes referred to as polymer microcomposites.
Thermoplastic polymers used in making reinforced polymer composites, such as those described above, include a wide range of thermoplastics, such as polyimides, polyethylene, polystyrene and copolymers thereof, polyamides, polycarbonates, polyetherimide and polyesters such as polybutylene terephthalate. These thermoplastic polymers are either amorphous or semi-crystalline and may be called flexible chain polymers, since individual monomer units in the polymer chain are free to rotate with respect to each other so that the polymer chain may assume a random shape.
Thermotropic LCPs are a relatively new class of polymeric materials which combine the advantages of melt processability and outstanding mechanical properties. Due to their rigid-rod molecular conformation and capability to form highly oriented crystalline structures, i.e., an ordered phase when subjected to shear above their melting point, they form products with properties similar to fiber reinforced composites. However, if the orientation of the polymer is in only one direction, such products are not suitable for applications requiring strength in more than one direction. Because of their rigid backbone structure with flexible spacer groups, commercially available TLCPs have far higher tensile strength and flexural moduli than conventional polymers.
Thermotropic LCPs can be processed in the melt state and they are capable of forming a highly oriented fibrillar structure when subjected to shear above their melting point. Methods for producing such highly oriented fibrillar structures are disclosed in U.S. Pat. Nos. 4,973,442; 4,939,325; 4,963,428; and 4,966,807 (hereinafter referred to collectively as the "CRD Patents"). The disclosure of each of these patents is incorporated herein by reference. A brief discussion of this methodology follows.
A schematic diagram of the process disclosed in the CRD Patents is shown in FIGS. 1A and 1B. A combination of shear and elongational flows during the extrusion process orients the TLCP polymers. This controlled orientation can be accomplished with the counter-rotating die shown in FIG. 1A that aligns TLCP molecules along at least two distinct axes within a single ply. The angle that the TLCP fibrils make with the longitudinal axis of the film is .+-.theta, where theta can be varied from near zero to over 50 degrees. By rotating the mandrels, a transverse shear flow is superimposed on the axial shear developed as the polymer melt is extruded through the die. It is possible to obtain films in accordance with the CRD Patents having a thickness ranging from about 0.0001 to 0.060 inches.
In the CRD Patents, the objective was to obtain extruded articles, such as films and tubular components, having optimized tensile strength, tensile modulus, coefficient of thermal expansion, and other properties related to in-plane stresses and deflections of the film. As disclosed in the CRD Patents, such properties can be controlled and enhanced by alignment, orientation and organization of the rigid thermotropic LCP molecules. In the technology disclosed in the CPD Patents, reinforcement is achieved by the LCPs in fibrillar form. This morphology was observed directly microscopically and indirectly through effect on mechanical properties.
In such methods, subsequent post-die processing enhances the orientation already present as the material exits the die. For example, post processing such as post-die draw in the transverse and/or machine direction can be performed on the extruded article to further optimize properties or obtain a finished product.
Because TLCPs form an ordered phase in the melt (hence, the name thermotropic), they have shear viscosities far lower than other polymers, This property gives them potential importance as a processing aid.
Thermotropic liquid crystalline polymers have received increasing attention in the scientific and technical literature as in situ reinforcements in polymer blends and microcomposites. The range of high performance thermoplastic flexible polymers blended with TLCPs include polyimides, polyamides, PES, PEI, PEEK, polycarbonate, PET, PPS, and polyarylace. The blending of thermoplastic flexible polymers and LCPs occur at various size scales down to the molecular level to form the systems referred to as polymer microcomposites (PMC).
The microstructure of a polymer microcomposite is similar to fiber-reinforced composites except that the fibers are at a micron to submicron scale. Blends of thermoplastic polymers and TLCPs are disclosed, e.g., in U.S. Pat. Nos. 4,386,174; 4,728,698; 4,835,047; and 4,871,817, the disclosures of which are incorporated herein by reference.
The potential advantages of blending thermoplastic matrix polymers with thermotropic LCPs are well recognized. Yet, despite the potential advantages of combining thermoplastics with TLCPs, traditional processing steps have failed to yield the optimal properties desired in blends. To achieve the optimal properties with such blends, processing techniques are used that permit the controlled orientation of the rigid-rod polymer in melt state and subsequent freezing in the desired morphology.
Although fibers and films of LCP blends have shown the most promise in terms of properties, they typically have consisted of a highly uniaxially oriented structure with correspondingly inferior transverse properties. This anisotropy is the bane of thermotropic LCP blends, limiting their use primarily to spun fibers. To extend the applications of thermoplastic flexible polymers/TLCP blends to two and three dimensional articles, the fibrillar orientation of the TLCP reinforcing phase must be controlled. Indeed, the processing of such blends into films, tubes, and other structures has been severely hindered by the difficulties encountered in con-rolling the orientation and CTE of the final product.
Until recently it had not been possible to form articles, such as films and tubes, comprising blends of thermoplastic flexible polymers and TLCPs and to obtain controlled multiaxial orientation of such articles. Such articles and methods of obtaining them are disclosed in application Ser. No. 07/678,080, filed Apr. 1, 1991, now abandoned. One such method involves use of a counter-rotating die (CRD) and the technology disclosed in the CRD Patents, supra.
In general, a multiaxially oriented article is produced which has a tailored CTE and comprises at least one thermotropic LCP and at least one thermoplastic flexible. The method comprises:
(i) extruding a melt of the polymer or polymers, under conditions which impart axial and transverse shear thereto to form a multiaxially oriented article; and PA1 (ii) maintaining the article under conditions to enable solidification of the orientation formed in step (i). PA1 Potentially lower product cost PA1 Wider range of performance (higher temperature, strength) PA1 Ability to form into articles of manufacture not possible with multi-layer films
The method may further comprise the step of subjecting the article to post-die draw in the axial and/or transverse direction between steps (i) and (ii). When it is desirable to increase the bend and fracture toughness of the article, e.g., the film or tube, the film or tube is stretched at above the Tg of the thermoplastic flexible polymer.
Co-pending Ser. No. 07/678,080 filed Apr. 11, 1991, now abandoned teaches that LCP-thermoplastic blends can be processed as disclosed in the CRD Patents to achieve fibrillar morphology and to orient the fibrils during processing to greatly improve mechanical properties with only small amounts of LCP (10% for example) in the blend.
A major problem currently exists in the packaging industry because of the relatively poor barrier properties of plastic materials used in films, bags, bottles, cars and ocher containers. Packaging materials have long since been developed with excellent barrier properties, but they do so with multiple layers, typically three to seven layers, including separate layers for oxygen and moisture barriers. Although special co-extrusion machinery has been developed to make such films, they are still perceived as being environmentally "unfriendly" because they cannot be recycled. Also, co-extrusion requires the use of secondary "tie" layers to bond the other layers together, and the machinery is generally more expensive to build and operate than equipment for extrusion and processing of single polymer materials.
Furthermore, it is not possible to recycle most of such multiple layer packaging materials, because the components of the multi-layers are irreversibly melted together during thermoplastic recycling. Plastic materials which can be recycled (such as polyethylene) are not very good barriers to gases such as oxygen, air and water vapor, and therefore cannot be used for long storage times. Accordingly, materials which combine excellent barrier properties with ability to recycle, creating a new generation of food and beverage packaging materials are being sought.